I think mattkb110 might be on the money especially so as it sounds like the previous owner had a mystery cam installed?, otherwise;

A15s naturally have a longer, thus weaker pushrod. I would be checking your oil supply to the rocker assembly which might have starved the rocker arms of lubrication making them bind into the rocker shaft and cease up. Sometimes they cease themselves so hard into the shaft that the don't even move.

Remove the rocker assembly and check all rockers are moving freely.

Also check your oil supply and pressure, I believe the previous recipient of the oil circuit is the cam assembly so be sure to check for pressure drop there as well.

I have heard of a very heavily sprung valve having enough pressure to bend the longer, weaker a15 pushrods, but that is most likely too much spring pressure.